1. Field of the Invention
The present invention relates to an apparatus and method for supplying direct current (DC) from the main of an alternating current (AC) line without connection of the main to the return side or common of the AC line or to safety ground.
2. Background to the Invention
Household and commercial power distribution wiring is typically locally controlled through simple mechanical wall switches placed in one line of a two line AC distribution system. Generally one line comprises two wires with the first wire being a hot feed into a box for the wall switch and the second wire connecting the box to the controlled load, e.g. a lighting fixture. Generally the other side of the load is connected to a neutral line or leg of the AC power supply called the common which does not return to the wall switch box.
It may be desirable to substitute electronic switching devices for the simple mechanical wall switches to provide automatic functions which the mechanical wall switch cannot support. An example of such a device is an electronic motion sensor for activating room lighting when a room is occupied. The electronic control circuitry for such a device requires DC power. Deriving the DC power from the AC line serves user convenience and is more economical than use of batteries for the electronic circuit or providing a household DC power system. As only one leg of the AC system is generally routed through the switch box, any local DC power supply using the household system must provide for supplying DC from a single line of the AC system, i.e. without connection to the neutral leg of the AC system. The power supply must also be maintained both when fixture is on and when it is off.
U.S. Pt. No. 4,874,962 ('962 patent) teaches one such power supply. The power supply component of the circuit of the '962 patent is connected between the hot leg of the AC system and earth ground. For a variety of reasons, electrical codes have long strictly limited leakage current to ground. In the '962 patent a neon lamp is included in the current path of the power supply circuitry to limit current to under 500 .mu.A, the code limit. Among the advantages of this arrangement are that no current need be drawn by the load to power the power supply circuitry. A disadvantage of the solution proposed by the '962 patent is that deliberate leakage of current to ground is not always desirable even if within code limits. The use of a neon lamp to limit current also adds to the expense of the circuit.
More commonly, DC power is tapped from an AC line by inductively coupling the DC power supply circuitry to the AC line or by connecting the power supply in series with the load in the AC line. In such an arrangement, the load must draw some current if DC power is to be available whenever needed. Of course a load is not normally drawing current when "off". In practice this means that current trickles through the AC line when the load is off. The AC circuit should be a low impedance circuit when the load is fully powered to avoid heat generation and power waste in the DC power supply and an extremely high impedance device when the load is off to minimize current in the line.
U.S. Pat. No. 4,713,598 (the '598 patent) teaches a DC power supply connected in series with a load in a leg of an AC building power distribution system. In the '598 patent a relay switch and the primary winding of a current transformer provide the main conduction path to the load when the load is powered for use. The position of the relay circuit controls the current path through the current transformer. When the relay is open, parts of or all of both the primary winding and the secondary winding and a capacitor are connected in series with the load. The combination of windings and capacitor are relatively high in impedance compared with the load reducing current in the line to a low level or trickle current and reducing the voltage drop across the load substantially. For practical purposes, the load is no longer on when a trickle current is being sent through the load. The trickle current is tapped from the current transformer, which now functions as a voltage dividing impedance. The tapped trickle current powers the DC supply. The current transformer is connected to rectification circuitry to provide the DC supply. The circuit of the '598 patent taps the AC line for power on every cycle of the AC current.
The problems presented by putting a DC supply circuit in series with the load in a AC line are well known. The circuitry tends to introduce additional noise on the line reducing power quality. The line always draws some power. The power supply circuit can reduce maximum voltage across the load, possibly reducing its efficiency and producing visible effects on lighting intensity.